Derek Knight shops at City Sports in Georgetown at 3338 M St. NW. The retailer has opened five locations inside the Beltway in eight years. (Evy Mages/For Capital Business)

It used to be that a T-shirt and pair of sweats was the outfit de jour at the gym. These days fitness apparel, much like workouts, have become more sophisticated. There are shirts with ventilation panels, pants with thermal lining and they all can be found in the Washington area.

Dozens of speciality stores, including Lululemon and Lucy, have sprung up around town to cater to a growing health-conscious population.

In 2011 alone, Modell’s Sporting Goods opened in Columbia Heights, Althleta debuted in Georgetown and City Sports took up an address in Silver Spring. Outdoor gear and apparel retailer Recreational Equipment Inc., or REI, is putting the finishing touches on a location in Woodbridge set to open later this year.

“The D.C. area has a young, affluent and highly educated population that is much more health conscious than other places in the country,” said Cushman & Wakefield broker David Dochter, who represented Athleta in the Georgetown deal. “It’s a perfect culmination of the characteristics that retailers and gyms want.”

Area demographics were a huge draw for Boston-based retailer City Sports, said the company’s executive vice president of merchandising, Michael Mosca. The company has opened five locations inside the Beltway in eight years.

Raymond Hrin looks for running shoes at City Sports. (Evy Mages/For Capital Business)

“This market has been good to us,” Mosca said. “We’re definitely open to additional locations if the opportunity is there.”

City Sports carries an assortment of indoor and outdoor workout gear. Mosca said attire for running and cycling are especially popular, though general fitness wear continues to fly off of the shelves.

Growing interest in a wide variety of athletic activities has kept the fitness apparel industry humming with an estimated $34.5 billion in sales in 2011, a 6.7 percent increase from the prior year, according to research firm IBIS World.

Speciality stores aren’t the only ones ringing up sales for workout wear. Big-box retailers Target and Wal-Mart sell discount lines, while women’s apparel stores, such as Victoria’s Secret, Bebe and Forever 21, now are offering athletic wear.

“It’s not just Nike or Adidas anymore,” said Janet Shim, a retail analyst at IBIS World. “Traditional women’s apparel stores are expanding their product lines to accommodate their customers.”

Despite the crowded field of contenders, Shim asserts that oversaturation is not eminent because “more people are wearing sports apparel as everyday clothing, and that’s a growth opportunity for retailers.” Brand loyalty, however, will be key to growth.